Your request may be difficult to fulfill, because all of 3D graphics are an illusion produced by programming mathematics algorithms in some language. Because OpenGL and DirectX have C/C++ APIs, the language used for those is going to be predominantly C/C++.

Maybe what you want is the mathematics without the programming? If so, you would want to start out with geometry and linear algebra, in order to learn the means of representing and working with 3D data types in equation and matrix/vector form.